This invention relates to improved ultrasound imaging and, more particularly, to an ultrasound imaging method which employs both harmonic frequency backscatter signals which result from non-linear tissue response and fundamental frequency backscatter signals to provide improved image presentations.
Researchers using contrast enhancing agents with diagnostic real-time medical ultrasound instruments have observed non-linear responses from both the contrast agents and from body tissues. A mode of system operation which has demonstrated this effect is the xe2x80x9csecond harmonic contrast agent modexe2x80x9d. In such mode, a low frequency ultrasound signal is transmitted and the received backscatter signals are processed and filtered in such a manner as to be most responsive to the second harmonic of the fundamental frequency of the transmitted signal. When certain contrast agents are in use, their strong non-linear response characteristics, usually associated with the destruction of microbubbles, results in a significant second harmonic energy signal being generated and imaged by the ultrasound system.
Even when contrast agents are not in use, ultrasound systems have been used in the second harmonic imaging mode. See, for instance, xe2x80x9cA New Imaging Technique Based on the Non-Linear Properties of Tissuesxe2x80x9d, Averkiou et al., (Proceedings of the 1997 IEEE Ultrasonics Symposium). Averkiou et al. report that the use of harmonic energy for image formation provides enhanced image features, e.g., delineation of the endocardial borders in the instance of echocardiographic imaging. However, not all aspects of the image are enhanced. Specifically, the tissue texture, contrast resolution, spatial resolution and low speckle content aspects of an acceptable ultrasound image are seen to degrade when imaging in the harmonic mode. Such image degradation is principally due to the fact that in order to operate in the harmonic mode, the transmit signal is lowered in its fundamental frequency and is often reduced in bandwidth. These changes concentrate a sufficient amount of ultrasound energy at the fundamental frequency to produce a detectable response at the second harmonic frequency, but cause degradation of certain aspects of the resulting image.
To overcome some of the problems which result during second harmonic imaging, Averkiou et al. suggest use of various image processing techniques. For instance, it is suggested that the speckle artifact can be reduced by various frequency compounding (or diversity) approaches, (e.g. such as shown in U.S. Pat. No. 4,561,019 to Lizzi et al.). The Lizzi et al. method employs an averaging of signals from two or more frequency bands to reduce the speckle artifact.
Thus, while it is known that harmonic backscatter signals which result from non-linear properties of tissues produce certain enhancements in ultrasound images, the resulting images are generally inferior to those images that are constructed from backscatter returns at the fundamental frequency of a transmitted ultrasound signal. Nevertheless, certain of the image features that result when harmonic imaging techniques are employed are useful to the clinician. Accordingly, improved ultrasound images will result if features of both fundamental and harmonic ultrasound imaging can be combined.
A method for processing ultrasound signal samples obtains enhanced images of a body structure through use of both harmonic and fundamental image processing. The method includes the steps of first obtaining a first plurality of signal samples from sensed backscatter signals that fall within a range of frequencies about a fundamental frequency of the transmitted ultrasound signal. Next, a second plurality of signal samples are obtained from backscatter signals that fall within a range of frequencies about a harmonic of a transmitted ultrasound signal. Control values are then derived from at least one of the first plurality of signal samples or the second plurality of signal samples. Those control values are then utilized to control the processing of the other of the signal samples to derive an improved ultrasound image. In a preferred embodiment, the control values are derived from the signal samples which result from sensed harmonic backscatter signals. Further, the control values are utilized to alter the processing of the fundamental signal samples (e.g., by variation of applied gain, filter frequency, filter bandwidth, etc.).